This invention relates to an optical delay circuit and, more particularly, to an optical delay circuit available for an optical exchanger incorporated in an optical communication system and a method used in the optical delay circuit.
A typical example of a prior art optical delay circuit is illustrated in FIG. 1 of the drawings. The prior art optical delay unit is broken down into an optical switching element 100 and a controller 101. The optical switching element 100 has a single input port and four output ports, i.e., the first output port a, the second output port b, the third output port c and the fourth output port d, and the input port is connected through four optical fibers (not shown) to the first, second, third and fourth output ports a/b/c/d. The optical delay element 100 is responsive to an instruction of the controller 101 so as to selectively transfer an optical signal from the input port through the optical fibers to the first, second, third and fourth output ports a/b/c/d.
An input optical signal line is connected to the input port, and output optical signal lines are respectively connected to the first, second, third and fourth output ports a/b/c/d. Time on the input optical signal line is divided into frames, and each frame is further divided into time slots. The first optical fiber does not introduce any time delay into the propagation to the first output port a. However, the other optical fibers introduce delay times different from one another. The second optical fiber introduces a time delay equal to a single time slot into the propagation from the input port to the second output port b, and the third optical fiber introduces a time delay twice as long as the time slot into the propagation from the input port to the third output port c. The fourth optical fiber introduces a time delay three times as long as the time slot into the propagation from the input port to the fourth output port d. The controller 101 instructs the optical delay element 100 to steer the optical signal to one of the first, second, third and fourth optical fibers.
An optical time switch is disclosed in Japanese Patent Publication of Unexamined Application No. 63-209395, and is illustrated in FIG. 2 of the drawings. The prior art optical time switch has an input terminal 201, and a wavelength converter 202 is connected to the input terminal 201. The wavelength converter 202 converts the wavelength of an optical signal to one of the different wavelengths xcex0, xcex1, xcex2, xcex3, xcex4, xcex5, xcex6 and xcex7. When a multiple optical signal is supplied to the wavelength converter 202 as a frame divided into plural time slots, the wavelength converter 202 converts the wavelength of the pieces of multiple optical signal in the plural time slots to the wavelengths xcex0, xcex1, xcex2, xcex3, xcex4, xcex5, xcex6 and xcex7, respectively. Between the input terminal 201 and an output terminal 204 are serially connected optical delay circuits 231, 232 and 233 which are similar in circuit configuration to one another.
An optical multiplexer 251, an optical delay line 261 and an optical demultiplexer 271 form in combination the optical delay circuit 231. The optical multiplexer 251 is of the type having a single input port and two output ports, and the optical demultiplexer 271 has two input ports and a single output port. The input port of the optical multiplexer 251 is connected to the wavelength converter 202, and two output ports thereof are connected to the optical delay line 261 and one of the input ports of the optical demultiplexer 271, respectively. The optical delay line 261 introduces a delay time 4T into the propagation from the optical multiplexer 251 to the optical demultiplexer 271, and supplies the delayed optical signal to the other of the input ports of the optical demultiplexer 271.
The optical delay circuit 232 also includes an optical multiplexer 252, an optical delay line 262 and an optical demultiplexer 272, which are arranged as similar to the optical multiplexer 251, the optical delay line 261 and the optical demultiplexer 271. An optical multiplexer 253, an optical delay line 263 and an optical demultiplexer 273 also form in combination the optical delay circuit 233, and are arranged as similar to those of the optical delay circuit 231. Thus, the optical delay circuits 231, 232 and 233 are similar in circuit configuration to one another. However, the optical delay lines 261, 262 and 263 introduce different delay times into the propagation of the optical signal from the associated optical multiplexers 251, 252 and 253 to the associated optical demultiplexers 271, 272 and 273.
A single bit of the optical signal occupies a time T, and each frame occupies a time period 8T. The optical delay line 261 introduces the delay time equal to 4T. The optical delay line 262 introduces the delay time, which is equal to a half of the delay time introduced by the optical delay line 261, i.e., 2T. The optical delay line 263 introduces the delay time, which is equal to a quarter of the delay time introduced by the optical delay line 261, i.e., T.
A multiple optical signal contains pieces of data information on eight channels A, B, C, D, E, F, G and H, and the pieces of data information on the eight channels A, B, C, D, E, F, G and H are respectively assigned to the eight time slots. Assuming now that the multiple optical signal is supplied to the input terminal 201, the wavelength converter 202 converts the eight pieces of the multiple optical signal respectively occupying the eight time slots to optical signals with the wavelengths xcex0, xcex1, xcex2, xcex3, xcex4, xcex5, xcex6 and xcex7. The optical signals are supplied to the series of optical delay circuits 231, 232 and 233. The optical signals are suitably multiplexed, delayed and demultiplexed, and are rearranged in a different order. The multiple optical signal at the output terminal 204 carries the pieces of data information put in a different order H, F, C, G, A, D, E and B, by way of example. Thus, the prior art optical time switch achieves an exchange of time slots.
The prior art optical delay circuit shown in FIG. 1 introduces the different delay times through the individual optical fibers. The prior art optical delay circuit shown in FIG. 1 outputs three delayed optical signals at the output ports b, c and d, and, accordingly, three optical fibers are incorporated in the optical delay element 100. When the prior art optical delay circuit is to be designed to produce n delayed optical signals, the optical delay element 100 requires n output ports and, accordingly, n optical fibers. The longer the delay time is, the longer the optical fiber becomes. If n is much greater than three, the optical delay element 100 becomes huge due to n optical fibers and, especially the nth optical fiber much longer than the first optical fiber. Thus, the problem inherent in the prior art optical delay circuit is the volume increased together with the number of delayed optical signals.
The prior art optical time switch differently retards the optical signals with different wavelengths. The pieces of data information are assigned to the different time slots, and are converted to the optical signals different in wavelength. Each of the optical delay circuits 231, 232 and 233 generates two optical signals. If the multiple optical signal occupies n1 time slots, the prior art optical time switch requires n2 optical delay circuits satisfying the relation of n1=2n2. As a result, if the multiple optical signal occupies a large number of time slots, the prior art optical time switch also becomes huge.
It is therefore an important object of the present invention to provide an optical delay circuit, which is still small under the conditions that an optical frame comprises a large number of time slots.
It is also an important object of the present invention to provide a method used in the optical delay circuit.
To accomplish the object, the present invention proposes to repeatedly use a waveguide as a delay line introducing unit delay time during generation of an output optical signal.
In accordance with one aspect of the present invention, there is provided an optical delay circuit for successively introducing delay times respectively equal to multiples of a unit time between parts of an input optical signal and an output optical signal, and the optical delay circuit comprises a first optical multiplexer having a first input port successively supplied with the parts of the input optical signal representative of pieces of data information at a predetermined wavelength and a second input port successively supplied with intermediate optical signals each representative of none of or at least one of the pieces of data information at another wavelength different from the predetermined wavelength in synchronism with the parts of the input optical signal and an output port for outputting a multiplexed optical signal representative of the piece of data information on one of the parts of the input optical signal at the predetermined wavelength and the aforesaid at least one of the piece of data information at the aforesaid another wavelength, an optical demultiplexer having an input port connected to the output port of the first optical multiplexer and plural output ports for outputting plural demultiplexed optical signals representative of the aforesaid one of the pieces of data information at the predetermined wavelength and the aforesaid at least one of the pieces of data information at the aforesaid another wavelength, a first wavelength converter having input ports respectively connected to the output ports of the optical demultiplexer and converting the plural demultiplexed optical signals to converted optical signals at the aforesaid another wavelength and yet another wavelength different from the predetermined wavelength and the aforesaid another wavelength, an optical switching unit having input ports respectively connected to output ports of the first wavelength converter and first output ports connectable to the input ports and second output ports also connectable to the input ports and responsive to an instruction so as to selectively connect the input ports to the first output ports and the second output ports, a second wavelength converter having input ports selectively connected to the first output ports and one of the output port of the first wavelength converter and converting the converted optical signals to restored optical signals respectively representative of the pieces of data information at the predetermined wavelength, a second optical multiplexer having input ports respectively connected to the second output ports, and successively producing the intermediate optical signals from the converted optical signal or the converted optical signals selectively supplied from the second output ports, a third optical multiplexer having input ports respectively connected to output ports of the second wavelength converter and producing the parts of the output optical signal from the restored optical signals, a waveguide connected between an output port of the second optical multiplexer and the second input port of the first optical multiplexer and introducing a delay time approximately equal to the unit time into the propagation of each of the intermediate optical signals from the second optical multiplexer to the first optical multiplexer and a controller storing pieces of control data information representative of the delay times and checking the pieces of control data information to see whether or not the optical loop consisting of the first optical multiplexer, the optical demultiplexer, the first wavelength converter, the optical switching unit, the second optical multiplexer and the waveguide retards the pieces of data information by the delay times, respectively, so as to instruct the optical switching unit to selectively change the optical connection from the first output ports to the second output ports.
In accordance with another aspect of the present invention, there is provided a method for introducing time delays equal to different multiples of a unit time between parts of an input optical signal and parts of an output optical signal comprising the steps of a) multiplexing one of the parts of the input optical signal representative of any one of pieces of data information with an intermediate optical signal representative of none of or at least one of the pieces of data information at another wavelength for producing a multiplexed optical signal representative of the aforesaid any one of the pieces of data information at the predetermined wavelength and the aforesaid none of or at least one of the piece of data information at the another wavelength, b) demultiplexing the multiplexed optical signal into plural demultiplexed optical signals representative of the aforesaid any one of the pieces of data information at the predetermined wavelength and the aforesaid none of or at least one of the pieces of data information at the another wavelength, c) converting the demultiplexed optical signals at the predetermined wavelength and the aforesaid another wavelength to converted optical signals at the aforesaid another wavelength and yet another wavelength, d) checking the converted optical signals to see whether or not any one of the converted optical signal is delayed by associated one of the delay time, e) transferring the aforesaid any one of the converted optical signals to a second wavelength converter with a positive answer at the step d) and to a second multiplexer with a negative answer at the step d), f) multiplexing the aforesaid any one of the converted optical signals transferred at the step e) with another of the converted optical signals by means of the second multiplexer for producing the intermediate optical signal, g) propagating the intermediate optical signal from the second multiplexer through a waveguide to the first multiplexer for introducing a unit delay time equal to the unit time during the propagation and h) converting the aforesaid any one of the converted optical signal to a restored optical signal at the predetermined wavelength by means of the second wavelength converter without execution of the steps f) and g) for producing one of the part of the output optical signal when the answer at the step d) is positive.
In accordance with yet another aspect of the present invention, there is provided an optical delay circuit for selectively introducing delay times different from one another between pieces of input optical data and pieces of output optical data comprising an optical path circulating the pieces of input optical data and introducing the delay times during the circulation of the pieces of input optical data, an output optical means for outputting the pieces of output optical data, an optical switching means inserted into the optical path and connected to the output optical means and a controller storing pieces of control data respectively representative of the delay times to be introduced and checking the pieces of control data to see whether or not the optical path retards any one of the pieces of input optical data by the delay time assigned thereto, when the aforesaid any one of the pieces of input optical data is delayed by the delay time, the controller instructs the optical switching means to transfer the aforesaid any one of the pieces of input optical data to the output optical means.